Electric heating element



Feb. 13, 1968 B, EDWl ETAL 3,369,209

ELECTRIC HEATING ELEMENT Filed Fb. 5, 1965 United States Patent 3,369,209 ELECTRIC HEATING ELEMENT Bjiirn Edwin, Asvagen 22, and Thor Hegbom, Tegelbruksvagen 19, both of Hallstahammar, Sweden Filed Feb. 5, 1965, Ser. No. 430,642 Claims priority, application Sweden, Feb. 7, 1964, 1,538/ 64 5 Claims. (Cl. 338--238) ABSTRACT OF THE DISCLOSURE This invention relates to an electric heating element including an outer sheath, at least one resistance wire housed within said sheath and surrounded by magnesia powder, and said sheath containing, or alternatively, said powder containing ferrous oxide, nickel oxide, cobalt oxide or copper oxide in an amount sufficient to preclude substantial discoloration of the magnesia powder when the elementis sealed in a gas-tight manner and heated to an elevated temperature.

The present invention relates to heating elements and relates more particularly to an electric heating element to be used at operation temperatures above 700 C. in air and comprising an outer metallic sheathing which houses one or more resistance elements embedded in a magnesia filling in the absence of materials of the kind that easily give ofl oxygen.

Basically, such heating elements are manufactured by starting, for example, with a tubular blank and consisting of a heat resistant material. The tube is placed vertically and filled with a central coil of resistance wire around which there is placed as electrical insulation a suitable material such as magnesia powder. During this procedure the tube is vibrated to ensure a uniform distribution of the insulating material. Then, the tube with its contents is compressed from a diameter, for example, 9.5 mm. down to 8 mm. to ensure that the coil is held firmly and that the heat conductivity of the powder is improved. Finally, end seals are applied, if desired. Such an element may be used straight with or without a reflector for use in heating rooms, for example, or it may be wound up in a spiral to be used as a cooking plate in a well known manner. Naturally, it is very essential to maintain a proper insulation under all conditions, and it is this problem with which the instant application is concerned.

With electric heating elements of the above-mentioned kind which, for convenience, will be referred to as tubu lar elements in the following specification, it has in some instances been found that the insulation resistance has deteriorated with time. In such cases, the magnesia has been considerably discolored and blackened, as compared to the magnesia filling of tubular elements where no deterioration of the insulation resistance has been encountered. It has 'also been ascertained that the discoloring is not of the type which would emanate from organic compounds, although the reaction process in connection with the discoloring has not been clearly established as yet. It should be noted that the term discoloring, as used herein is intended to include not only the color-change per se, but moreover the substantial reduction of the electrical insulation resistance inherent with such colorchange.

It has been found that one of the reasons for the discoloring may be that the atmospheric conditions Within the tube are successively changed, as evidenced on the one hand by a pressure reduction, and on the other hand by an alteration of the gas composition such as decrease of the oxygen content by an oxidation of the sheathing and/or of the resistance wire. Thus, it has been found,

for example, that a tube material of the most commonly utilized type, comprising nickel-chromium-iron alloys, has a greater deteriorating effect on the insulating resistance than a tube material of a normal carbon steel. Furthermore, it has been ascertained that while the composition of the resistance material contributes to the deterioration, this factor appears to be of less relevance inasmuch as the total deteriorating effect may be reduced by a suitable choice of sheathing material. The increased significance of the outer sheathing will be recognized on comparing the relatively large area of the same in respect to the resistance wire housed therein.

In an attempt to afford a closer study of the conditions for discoloring, tubes of various compositions have been filled with magnesia powder and sealed in a gas-tight manner by welding the ends thereof, whereafter the tubes have been heated up to 1125 C. for five hours. The results of this study showed that magnesia in tubes of alloyed materials was strongly discolored. Therefore, it would appear that the discoloring process is related to the ability of the sheathing material to combine with oxygen thereby influencing the oxygen partial pressure within the tube.

For 'an even closer study of the conditions of discoloring, a plurality of carbon steel tubes were filled with magnesia powder along with various chemical additives having different abilities to combine with oxygen at the test temperature. These tubes were also sealed in a gastight manner, and heated to 1125 C. for five hours. Some of the additives were found to cause discoloration, whereas others did not. As illustrative, and not to be considered as limiting, it might be mentioned that carbon, manganese, silicon, and chromium gave rise to a discoloring, while iron, nickel and cobalt did not. These tests indicate that the critical oxygen partial pressure for commencing the discoloration phenomenon at the said conditions would be somewhere between 10- and 10* atmospheres.

It will be noted that in the tests recited above, the tubes had been sealed in a gas-tight manner. In practice, this would not be the case with tubular elements, but end seals of various designs have been used in 'an attempt to prevent as far as possible the entrance of air and moisture into the tube. Furthermore, in actual practice, the magnesia filling is strongly compressed which, in and of itself, constitutes a considerable barrier to air penetration. Therefore, the commercially manufactured tubular elements may be equated to the test tubes in this respect;

A primary object of the present invention is to prevent as far as possible the reduction of the electric insulation resistance of magnesia powder which in certain cases arises in tubular elements after extended periods of heating to elevated temperatures. Consistent therewith, the in stant inventive concept provides that the sheathing should be made of a material such that, if a tube of the same with a magnesia powder therein is sealed in a gas-tight manner and heated to 1125 C. for five hours, no substantial discoloration of the powder occurs. To this end, it is also contemplated to dispose within the sheathing one or more oxides or other oxygen-containing compounds which are capable of preventing such a discoloration of the powder, either in the form of a superficial layer on the inner surface of the sheathing or as a compound added to the powder. In other words, this invention utilizes two slightly different methods for achieving the intended result, that is, the use of a suitable sheathing material and/ or, the addition of a material which would result in maintaining the oxygen partial pressure above the critical limit at which the magnesia powder is discolored.

considered as suitable for use as the tube material. To

clearly establish whether or not this would also be the case in actual practice, two tubular elements were prepared, one of which was provided with a sheathing of carbon steel whereas the other was provided with a sheathing of a nickel-chromium-iron alloy. Both elements were filled with magnesia powder, and a resistance element formed of a Kanthal-alloy (Reg. Trademark) (note the example for details of the composition of one such alloy) was used. The elements were heated to 1100 C. in a furnace, and the steel tube was protected against oxidation by maintaining a protective gas atmosphere in the furnace. By a continuous monitoring of the insulation resistance, it was ascertained that the carbon steel sheathing had its electric insulation resistance considerably less deteriorated than the alloy sheathing. However, as the above discussed deterioration of the insulation values are of practical significance only with respect to elements at temperatures above 700 C. in air, it will be realized that steel tubings per se may not be adopted without special precautions.

In view of this fact, it is proposed according to one embodiment of the present invention, to use as a sheathing material a combination of two metals or alloys in such a manner that the innermost part of the sheathing is made of a material with the recited property of being able to prevent discoloration, such as carbon steel, copper or nickel, whereas the outermost part of said sheathing is made of a material with the required heat resistance at elevated temperatures in air.

Sheathings of this general kind-may be manufactured in a plurality of different ways, such as forming the tubes from strips of compound material. Also, the sheathings may be formed from two tubes one introduced into the other. Another method contemplated by this invention is to apply, such as by flame spraying, iron, an alloy, an oxide or another oxygen-containing composition on to a strip or the inner surface of a tube, out of which the sheathing is then made.

As set forth above, it might also be possible to prevent discoloration by adding to the magnesia powder one or more oxides or other oxygen-containing materials, the oxygen partial pressure of which at the operating temperature would be above the limit at which deterioration is encountered. Examples of such materials are FeO, CoO, NiO and CuO.,Such additives may be introduced separately into the sheathing or, alternatively, they may be formed by oxidation of an inner portion of the sheathing.

It should be noted that Swedish Patent No. 152,930 discloses an addition of certain components having the ability to give off oxygen. However, in accordance with these teachings, the oxides which are used readily give off oxygen and thus have a very essential influence on the atmosphere within the tube in such a manner as to render the same mainly oxygen. This ensures the formation of a superficial protective coating on the resistance wire and/ or the inner surface of the tube. In this connection it should be noted that tubular heating elements, after filling and compression thereof, are annealed at temperatures above 1000 C., under which condition the said oxygencontaining additives give off their oxygen content. As distinguished from the Swedish concept, the present invention contemplates oxides of quite another character such as FeO, CoO, NiO, and the like, which may not be considered as giving off oxygen readily but which nevertheless would be able to increase the oxygen partial pressure in an enclosed space above the above-discussed critical :value at which a discoloration of the magnesia powder occurs. As an example, it should be noted that at 1125 C. the oxygen partial pressure of the materials mentioned in the Swedish patent is considerably higher than 1 atmosphere, whereas the oxides as now contemplated have oxygen partial pressure values of 10- and 10- atmospheres, respectively.

The instant inventive concept will be best understood by reference to the drawing wherein:

FIGURE 1 is a side elevational view of one form of heating element in accordance with the instant inventive concept, parts being in section for illustrative clarity;

FIGURE 2 is an enlarged longitudinal cross-sectional view of a portion of an electric heating element in accordance with one embodiment of this invention; and

FIGURE 3 is an enlarged longitudinal cross-sectional view of a portion of an electric heating element according to another embodiment of this invention.

Like reference characters refer to like parts throughout the several views of the drawing.

Referring now to the drawing, an electric heating element in accordance with the instant inventive concept is shown generally in FIGURE 1 and designated by the reference numeral 10. This element comprises basically an outer tubular sheathing 12 with at least one resistance coil 14 housed therein embedded in magnesia powder 16, end plugs 18 sealing the sheathing 12 in a gas-tight manner. The terminals 20 of the resistance coil 14 extend outwardly from the sheathing 12. The sheathing 12 is formed of a material which is resistant to heat at temperatures above 700 C. in air and the resistance coil 14 has an operating temperature of up to about 1050 C.

In the embodiment of the instant invention shown in FIGURE 2, the sheathing is formed of an outer layer 12a and an inner layer 12b, the former being made of a material resistant to heat at 700 C. in air such as a nickel-chromium-iron alloy, and the latter including an oxygen-containing material. While the drawing shows two distinct layers, it is to be understood that the inner layer 121) may merely be a portion of the sheathing which incorporates as an additive the oxygen-containing material. This material may be, for example, iron oxide, nickel oxide, cobalt oxide, copper oxide and the like. Although the material in its oxide form may be either incorporated as an additive or as a layer in the sheathing, it may take the form of an oxidizable material which during the formation treatment is heated to such a temperature as to oxidize the same thereby forming the oxygen-containing material in situ.

Alternately, instead of including the oxygen-containing material in the sheathing itself, an oxygen-containing powder may be added to the magnesia powder as illustrated in FIGURE 3 wherein the magnesia powder is identified as 16a and the oxygen-containing powder is identified as 1612. Once again, the oxygen-containing material may be either incorporated per se or formed in situ as described with reference to FIGURE 2.

It will be understood from the above description of the drawings that the significant factor in regard to this instant invention is the incorporation of an oxygen-containing material within the element either as a portion of the outer sheathing or as an additive to the mangesia powder which material has the characteristic of functioning to preclude any substantial discoloration of the magnesia powder when the element is sealed off in a gas-tight manner and heated to a relatively high temperature such as 1125 C. for an extended period such as five hours. This oxygen-containing material maintains the oxygen partial pressure above the critical limit described hereinbefore at which the magnesia powder tends to deteriorate.

In order to further facilitate an understanding of the instant inventive concept the following example is setforth which, it is to be understood, is to be interpreted merely as illustrative and not in a limiting sense.

Example A tubular element having a compound sheathing of 0.65 mm. wall thickness was chosen. This compound sheathing had an outer layer of 0.45 mm. thickness composed of Nikrothal 4R (Reg. Trademark), this ma terial being an alloy of the composition 33% Ni, 20% Cr, the balance essentially being iron, and an inner layer of 0.20 mm. thickness composed of carbon steel, containing 0.1% C, 0.2% Si, and 0.4% Mn, the balance essentially iron. Originally, the outer diameter of the tube was 9.5 mm. The tube was filled with magnesia powder, and as a resistance coil an 0.4 mm. diameter wire of Kanthal DSD (Reg. Trademark) was used, this material being an alloy of the composition about 23% Cr, 4.5% A1, 0.7% Co, 0.04% C, the balance essentially iron. The coil diameter was 2.9 mm. After filling, the tube was compressed from 9.5 down to 8.0 mm. outer diameter, and was then equipped with end seals. This tubular element was heated to 1125 C. for five hours, and after having been opened, no discoloring of the magnesia powder could be observed.

What is claimed is:

1. In an electric heating element including an outer sheathing, at least one resistance wire housed therein, and magnesia powder embedding the resistance wires, the improvement which comprises said sheathing housing therein a material precluding substantial discoloration of the powder when the element is sealed in a gas-tight manner and heated to 1125 C. for five hours, said material being selected from a group consisting of ferrous oxide, nickel oxide, cobalt oxide and copper oxide and being present in an amount sufficient to provide an oxygen partial pressure at 1125 C. in the order of from about 10 to about 10- atmospheres.

2. An electric heating element in accordance with claim 1 wherein the outer surface of the sheathing is formed of a material which is resistant to heat at temperatures above 700 C. in air.

3. In an electric heating element including an outer sheathing, at least one resistance wire housed therein, and magnesia powder embedding the resistance wires, the improvement which comprising the addition to the powder of a material precluding substantial discoloration of the powder when the element is sealed in a gas-tight manner and heated to 1125 C. for five hours, said material being selected from a group consisting of ferrous oxide, nickel oxide, cobalt oxide and copper. oxide and being present in an amount sufficient to provide an oxygen partial pressure at 1125 C. in the order of from about 10 to about 10- atmospheres.

4. An electric heating element in accordance with claim 3 wherein at least the outer surface of the sheathing is formed of a material which is resistant to heat at 700 C. in air.

5. An electric heating element in accordance with claim 1 wherein said sheathing is formed of a composite, two-layer material and said material is housed in the inner layer.

References Cited UNITED STATES PATENTS 2,360,267 10/1944 Osterhel-d 338-268 2,975,262 3/1961 Schnick 219-270 2,280,515 4/1942 Ridgway et a1. 106-60 2,280,516 4/ 1942 Ridgway 106-60 X 2,816,200 12/1957 Midge 338-238 X 3,201,738 8/1965 Mitofi 338-238 1,763,117 6/1930 Woodson 338-238 2,036,788 4/1936 Abbott 338-238 2,703,355 3/1955 Hagglund 338-238 2,767,288 10/ 1956 Lennox 338-238 RICHARD M. WOOD, Primary Examiner.

VOLODYMYR Y. MAYEWSKY, Examiner. 

